Elimination of corona in alternators



Dec. 16, 1930. c. F. HILL ELIMINATION OF CORONA IN ALTERNATORS Filed April 11, 1927 INYVENTOR Char/e; F H/'//.

AT'TORNEY Patented Dec. 16, 1930 UNITED STATES PATENT OFFICE- CHARLES F. HILL, OF WILKINSBURG, PENNSYLVANIA, ASSIGNOR TO WESTINGHOUSE ELECTRIC & MANUFACTURING COMPANY, A CORPORATION OF PENNSYLVANIA.

ELIMINATION OF CORONA IN ALTERNATORS Application filed April 11,

My invention relates to means and methods foreliminating corona on insulated conductors and it has particular relation to armature conductors lying in the slots of highvoltage, alternating-current machines.

Various attempts have been made heretofore to obviate the harmful effects of corona on the armature conductors inside of highvoltage machines, as by using mica insulation which was intended to withstand the chemical and thermal effects of the corona, the use of metal-foil strips wrapped spirally around the outside of the insulated armature conductor and grounded at one point to the metal laminations, and the use of means embedded in the insulation for controlling the distribution of the voltage stresses therein.

All of the above mentioned methods have been defective in that they have failed to reach to the fundamental cause of corona, besides being open to other objections, such as mechanical weakness, unreliability, and the introduction of hazards fully as great as the corona itself. Corona has not been a problem on the end turns lying outside of the armature core.

According to my invention, I coat the insulating covering of the'portions of the armature conductors lying within the slots of the magnetic core with a closely adhering highresistance conducting coating which is grounded at at least one point and may be,

and preferably is, grounded at manypoints to the iron laminations of the slots, said coating having sufiicient conductivity to main-- tain the ground potential of the laminations at all points, and, at the same time, having a sufiiciently high resistance to limit the circulated currents to a very small value.

Another object of my invention is to provide a practical means for embodying the corona-preventing means in the slots of a dynamo-electric machine,'with a good contact between the conducting resistance coating and the side walls of the slots.

1927. Serial No. 182,612.

With the foregoing and other objects in View, my invention consists in the structures, combinations and methods of building machines, which are hereinafter more clearly described and pointed out in the claims, with reference to the accompanying drawing, wherein; I

Figure 1 is a fragmentary longitudinal sectional view showing my improved armature conductor lying in the slot of a dynamo-electric machine, the section plane being indicated by the line 1-1 in Fig. 2."

Fig. 2 is a fragmentary transverse sectional view on the plane indicated by the line II II, in Fig 1, and i Fig. 3 is a similar view indicating a step in the process of manufacture.

The phenomenon of corona is a gaseousionization phenomenon. A certain proportion of all of the molecules in any gas are al- Ways ionized, due to the presence of ultraviolet radiations and other phenomena. The molecules of the gas, asis well known, are in constant n'iovement, colliding with one another and rebounding as a result of the thermal activity of the gas, the temperature being, in fact, a measure of such movement. In any gas, under given conditions of com pression, there is a fixed mean free path of vibration of the molecules, which is the average distance which one molecule travels before it collides with another molecule. When an electrostatic potential gradient exists in the gas, as a result of the presence of two spaced conductors having different electrical potentials, the molecules which are ionized are given an additional acceleration which is added to their heterogeneous movements, due to collisions, and, if the potential gradient is sufliciently high, a charged or ionized molecule will be accelerated to a suific'iently high speed, during its free path of movement, to ionize the molecule with which it collides, so that ionization by collision is produced in the gas as a result of the electrostatic potentialgradient. This phenomenon is known as corona.

The effect of corona in air is to produce J ozone, nascent oxygen and one or more of the oxldes of mtrogen, wh1ch,1n the presence of water or water vapor, become nitrous and nitric acids, particularly the latter. In a dynamo-electric machine, which is running with certain relatively high internal temm peratures, with the water vapor rather carearmature conductors and gradually eat the same way.

It will be understood, from the foregoing discussion, that corona is a gas-phenomenon and is not really present in solid dielectrics.

In fact, ionization cannot be produced in a solid dielectric until substantially the breakdown voltage of the dielectric is reached, at which time the dielectric fails, so..that it may be said that corona in the solid insula- 5 tion does not appear until the failure of the dielectric. As dynamo-electric machines are designed so that failure of the dielectric cannot occur, we may disregard the possibility of corona in the solid portions of the dieleco' tric.

In a dynamo-electric machine, although the insulated armature conductors are placed in the slot's of the laminated core as tightly as possible, it inevitably follows that there :5 are air spaces between the outside of the coil insulation and the side walls of the slot. Particularly is'this true at the junction points between the iron laminations, where air paths of greater length are to be found and where corona is more likely to occur by reason of the concentration of the electrostatic stresses at the sharp corners of the iron.

According to my invention, I apply, to the outer surfaces of the solid insulating covering which isprovided around the armature conductors, a tenacious, tightly adhering coating of a substance having a resistivity which may be as high as several thousand ohms per linear inch of the coil-side, said coating being so grounded to the core of the machine that it will 0 erate as a low-potential shield which is a solid, integral part of the insulation surrounding the armature conductor. I preferably utilize a conductor coating having a resistance of perhaps the order 'of three or four hundred ohms per linear inch, although still lower resistances may be tolerated, as will be apparent from a consideration of the losses involved by the circulating current therein, which will be presently pointed out. I I

It is necessary to consider what currents are generated in the conductive sheaths, as the sheaths are grounded to the frame at a plurality of points and hence there is a return path of relatively negligible resistance for all of the currents generated therein by the cutting of the magnetic fluxes.

The voltage generated. per unit len h in the armature conductors of dynamo-e ectric machines are more or less the same in all machines, because of limitations in the design thereof, so that we may figure on a generated voltage of from 1 volt per inch, in the armature conductors of low-speed machines, to about 2 volts per-inch in turbogenerators.

If we utilize a conducting coating having a resistance of four or five thousand ohms per inch in a machine in which the generated voltage is 1 volt per inch, the current induced in the conducting coating will be about one-fifth of a milliampere. If the resistance is 300 ohms per inch, the current will be about .003 amperes per conductor coating, and the PR loss in each conductor coating will be .003 X 300 or .003 watts per linear 1I1Cl1. In commercial dynamo-electric machines of large capacity, it is customary to dissipate about 0.5 watts in heat losses per linear inch for each armature coil. It will thus be seen that the losses introduced by my conductive coating are almost unmeasurable in a dynamo-electric machine.

Referring to the drawing, I have shown a dynamo-electric machine 5 having a laminated magnetic armature core 6 provided with slots 7, each of which contains two composite armature conductors 8 and 9, disposed one on top of the other and insulated from each other by a layer of asbestos board 10 or other insulating material. The conductors are retained in the slots by means of wedges 11. Each of the conductors 8 and 9 is composed of a large number of conductor bars 12 which are surrounded by a compact layer of insulation 13.

According to my invention, a thin coating 14 of resistance material is painted upon the outside surface of the insulation 13, the painted material being of such dilution that, when it dries, it will have a resistivity less than several thousand ohms per linear inch, or such other value as may be the limiting value for efiectively obtaining less than coronaproducing potential differences at all points of the coating. Preferably, my resistance coating has a resistivity of, say, 300 or 400 ohms per linear inch, although smaller values may be utilized, provided that the losses resulting from the circulating currents in the coating shall not constitute, a large part of the total losses dissipated per conductor in the machine, as pointed out hereinabove.

The most practical substance which I have found for coating the insulation 13 of the armature conductors is a colloidal solution of extremely finely divided carbon particles such, for example, as the substance known to the trade as aquadag, which may be painted upon the surface in any desired concentration or thickness, and which has the property of forming a thin, tenaciously adhering, hard coating which is practically an integral part of the solid material of the insulation 13, and which takes a hard glassy finish which cannot be rubbed off. The coating is extremely thin, of the order of a few microinches thick, and it is practically non-penetrating, when applied as a dilute paint thinly applied to the outside of the non-pervious paper wrapping which is commonly utilized 011 the insulation 13. The aquadag solution is easily applied and dries almost instantaneously. It is preferably applied to the outside of the active portion of the armature coil side, extending a short distance outside of the core of the machine, where the conductor emerges from the slot, as indicated at 14 in Fig. 1.

As apractical expedient in the manufacture and assembly of large dynamo-electric machines, it is quite desirable to use some sort of slot liner, which is commonly a paraffined fish-paper lining which is placed along the side and bottom walls of the slot. When the pre-formed coil-sides are placed into the slot, asindicated in Fig. 3, the liner is bent back over the edges of the slot to protect theconductor insulation from being cut or abraded. The parafiined surface of the paper also facilitates the sliding of the conductor into-the slot.

At it is practically impossible to have an absolute degree of uniformity in the widths of the coil-sides and slots, respectively, it has been customary, in placing the coil-sides in position, to try each coil-side before inserting it all of the way in the slot, and, if it is found to be at all loose, to remove it and to place one or more additional layers of fish paper in the slot in order to take up all lateral looseness.

When the bottom conductor 9 has been placed in position, the fish-paper lining 'is cut oil at a suitable point within the slot, the strip of insulation 10 is inserted, and the second coil-side 8 is placed in the top portion of the slot, with'a slot linerof its own.

Accordin to my invention, it is necessary for the sur ace of the resistance coating 14 on the insulation surrounding each of the armature conductors 8 and 9 to be in electricalcontact with the Walls of the slot at at least one point, and, preferably, for safety reasons and for uniformity in assembly, at many points along the length of the conductor; For the above noted purpose, the simplest plan is to utilize slot linings 16 for only the sides of the slots and utilizing no paper at the bottoms, so that, when the first armature conductor 9 is placed in position, the conducting high resistance coating 14 on the surface of its insulation will come into contact with the bottom of the slot and will thus be effectively grounded.

For the top armature conductor 8 in each slot, several expedients are available for effecting the ground connection between the conducting high resistance coating 14 and the slot walls. The method which is at present preferred by me is to cut off the slot'lining 16 of the bottom conductor 9 immediately above the top of said conductor, when it is in place, as indicated at 17 in Fig. 2, and place the top conductor 8 either immediately on top of the bottom conductor 9 or, preferably, to utilize the insulating spacer 10 previously described, but coated or impregnated with some conducting high-resistance material, which may be the same as that utilized for coating the surfaces of the conductors, as indicated by the line 18, so that the coating 14 on the insulation of the upper conductor 8 is electrically connected to the coating 14 on the insulation of the lower conductor 9,

. the slot liner 16 utilized for the upper condesired material laid along the sides of the slot when the armature conductor is inserted in place;

v The slot-closing wedge 11 may be either the conventional insulating wedge heretofore utilized, or it may be coated with the same resistance coating, as indicated by the line 19. As a substitute for, or as an addition to, the means hereinabove described for securing the conductive connection between the slot walls and the conducting high-resistance coatings 14 on my armature conductors, I may utilize a fish-paper liner, on one or both of the sides of the slot, which has beenprcviously provided with a number of tiny holes and impregnated with some conducting substance, such as the aquadag previously mentioned, so that a conductive connectionis effected by means of the conductive substance extending through these tiny perforations. In this manner, the effectiveness of the slot lining, by reason of its mechanical strength, insulating properties and lubricating properties, and as a filler for preventing any lateral looseness of the armature conductor, is not affected, particularly if one side of the slot is left with only one thickness of such perforated impregnated paper, any additional thicknesses which may be necessary tofill up the slot being applied on the other side.

Heretofore, the corona which has existed in the minute air gaps, which have been exaggerated in order to be illustrated at all, as shown at 20 in the drawing, have practically destroyed the slot linings, after several months of service of the machine, The outside paper insulation surrounding the arma ture conductors was then attacked, and, gradually, it, also, was eaten away, both by reason of the corona and byreason of the mechanical vibration incident to the looseness produced by the destruction of the slot lining.

It will be observed that, by my invention, I have provided a means for absolutely preventing the occurrence of corona, while, at the same time, introducing no conductor, such as the metal foil heretofore occasionally used, which would, of itself, introduce hazards, owing to its extremely good conductivity, so that it had to be carefully insulated from the slot at all points except one, and which was subject to being broken and thus introducing an additional hazard by reason of the concentration of the electrostatic field at the broken portion thereof. Moreover, I utilize a coat ing which is substantially an integral part of the solid insulation surroundin the arm ature conductor, as distinguished rom a foil which is merely wrapped therearound with tiny air spaces therebetween. In fact, an .essential feature of my invention is that there shall be no air spaces between the coating and the outer surface of the insulation.

While I have described my inventionin a preferred form in which slot liners are utilized, it will be understood that my invention is not limited .to such construction, and that my conductors may be placed directly in the slots without any lining at all, if so desired.

While I have described my invention, also, with reference to a certain theory of opera-. tion, I intend such explanations to be suggestive only and do not care to be restricted to-any particular theory of the phenomenon of corona.

The foregoing and other changes and variations in the application of my invention may be made by those skilled in the art without departing from the spirit thereof and I desire, therefore, that the following claims shall be given the broadest interpretation consistent with their language and the prior art.

I claim as my invention:

1. The combination with the core of a dynamo-electric machine, of an armature conductor embedded therein and having solid insulation therearound and a coating of a conductive hi h-resistance material integrally united to the surface thereof entirely cover-,

ing the portion which is intended to lie within the armature core with which the conductor is to be used, said coating havin sufficient conductivity to permit the flow 0 currents such as to introduce appreciable losses, but the resistance of said coating being sufficiently high that said losses are very small as compared to the ordinary copper losses in the armature.

2. The combination with the core of a dynamo-electric machine, of an armature conductor embedded therein and having solid insulationtherearound and a coating of a con ductive high-resistance material intimately associated with the surface thereof over the ,cuited through a portion which is intended to lie within the armature core with which the conductor is to be used, the resistance of the conductive high-' tion between said coating and said body, I

whereby the potential gradient in said air gap is insufiicient to cause ionization by collision.

LA high-voltage dynamo-electric machine having a slotted core, insulated electrical conductors lying in the slots of said core and having minute air gaps between the walls of the slots and the surfaces of the insulating material surrounding said conductors, high-resistance coatings adhering to said surfaces, the resistance of the coatings, in ohms per inch of conductor length, being of the order of a number not having a small number of digits times 10 and electrical connections between said coatings and said core, whereby the potential 'gradients I in said minute air gaps are insufficient to produce ionization by collision. v

5. The combination with the slotted armature core .of a dynamo-electric machine having an insulated armature conductor lying in a slot thereof, the potential gradient between the insulated armature conductor and the walls of said slot being normally sufficient to produce ionization by collision in any air gap therein, of a high-resistance conductive coating integrally united to the surface of the insulation surrounding said conductor, and electrical connections between many points of said coating and said slot walls, whereby the potential gradients in the minute air gaps therebetween are insufiicient to produce ionization by collision, said coating introducing small PR losses but the resistance of said coating bein sufiiciently high to prevent material PR osses even if short-cirpath of relatively small re-- sistance.

6. A high-voltage dynamo-electric machine comprising an electrical conductor having an insulating covering and a coating containing a deposit from a colloidal solution of carbon on the outer surface of said insulating covering, said coating having 'a resistance, when expressed in ohms per inch of conductor length, of the order of a .number having a small number of digits core and having minute gaps, between the walls of the slots and'the surfaces of the insulating material surrounding said conductors, high-resistance conductive coatings adhering to said surfaces, and electrical connections between said coatings and said core, the conductivity and resistance of said cpatings being such that the current induced therein is of the order of a milliampere,

whereby the ,potential gradients in said minute air gaps are insufficient to produce corona.

In testimony whereof, I have hereunto subscribed my name this 5th day of April, 1927.

. CHARLES F. HILL. 

